Pet with the highest attack

[Lisq]

Which pet has the highest attack?

[Hellomynameis]

Lil' Ragnaros has 374 power. But if you mean what ability does most damage in one round, or the highest two-round damaging ability, then you'd have to round up all high hitting abilities and compare them to the maximum power obtainable for pets with said ability, and work it out that way.